The present invention relates to a new and improved construction of theodolite for tracking and measuring a flying target or object--hereinafter simply referred to as a flying target--which is of the type comprising a telescope equipped with a telescope tube and composed of a mirror objective and a collimator. The telescope is mounted to be rotatable, about a horizontal axis, relative to a theodolite housing, and the theodolite housing, in turn, is mounted to be rotatable, about a vertical axis, in a socket or pedestal construction. The collimator projects to infinity a real image produced by the mirror objective and thus delivers an afocal beam of light which is infed by means of a first, partially permeable mirror surface to a television camera and to at least one other image evaluation device.
Now in German Pat. No. 1,802,120 there is disclosed a theodolite which is equipped with a telescope of the previously described type. The afocal beam of light is selectively delivered to one of a number of image evaluation devices by means of a mirror. According to a further development of such state-of-the-art theodolite the mirror is replaced by a partially permeable mirror, so that the beam of light can be simultaneously delivered to at least one television camera and further image evaluation device, for instance, a film or photocamera. Moreover, from the aforementioned prior art German patent it is also known to employ a partially permeable mirror for fading-in the image of a crosshair at the beam of light.
However, the heretofore known prior art construction of theodolite is associated with certain notable drawbacks. Firstly, this known arrangement results in the image of the crosshair being faded-in at all of the image evaluation devices. While this in itself is not disturbing as long as the television image only serves for observation, when using the television camera for controlling the target tracking (TV-tracking) it is however necessary to suppress the image of the crosshair. Additionally, the heretofore known arrangement is structured such that the rotational movement of the telescope tube about its horizontal axis of rotation causes rotation of the image of the television camera. When using the television camera for controlling the target tracking it is therefore necessary to either provide a coordinate transformation or a compensating measure, such as derotation for the image rotation. Due to this construction the equipment becomes more complicated and expensive.
Finally, it has been found that a partially permeable mirror, while constituting a simple optical element, does however have its light permeable or transmission characteristics vary in a rather complicated manner during its rotation. With the heretofore known arrangement the continuous TV-target tracking in combination with the possibility of switching between a number of cameras, requires that the TV-camera receive the beam of light which has been passed by the partially permeable mirror. It has been found that during switching by means of a good mechanically realized movement of the partially permeable mirror there cannot be avoided fluctuations of the light intensity at the TV-camera. This can however markedly impair the target tracking: the probability is great that shortly prior to an interesting phase of the target measuring there is switched from one camera, for instance a standard camera, to another camera, for instance a high-speed camera, which imparts a control surge to the automatic TV-target tracking, which, in turn, can result in loss of the target.
It is further to be mentioned that the delivery of the beam of light which has been passed by the partially permeable mirror to the TV-camera is unfavorable in terms of the coating of the mirror surface. What is desired is a division of the light beams, with about 80% going to the camera and 20% to the TV-camera. An 80% reflection requires a metallic coating, which is associated with high absorption losses.